Documentation for regex from raw element

Fixes #266.

library/src/compute/construct.rs

@@ -383,6 +383,10 @@ pub fn regex(
     /// escape sequences. To produce regex escape sequences that are also valid in
     /// Typst (e.g. `[\\]`), you need to escape twice. Thus, to match a verbatim
     /// backslash, you would need to write `{regex("\\\\")}`.
+    ///
+    /// If you need many escape sequences, you can also create a raw element
+    /// and extract its text to use it for your regular expressions:
+    /// ```{regex(`\d+\.\d+\.\d+`.text)}```.
     regex: Spanned<EcoString>,
 ) -> Value {
     Regex::new(&regex.v).at(regex.span)?.into()